Maraging steel is ultralow carbon-Ni steel or ultralow carbon-Ni—Co steel. It is a steel strengthened by precipitation intermetallic compounds of Ti or Mo, etc. on a matrix of tough martensite. It is tough and high in strength. It also possesses many other advantages not previously available such as good weldability and little change in dimensions by heat treatment. Therefore, maraging steel is used as a structural material in leading-edge technical fields such as space development, ocean development, atomic energy utilization, aircraft, and automobiles. Attempts are also being made to put it to use for a wide range or purposes in diverse fields such as pressure-proof vessels, tools, piston rams, and dies.
However, maraging steel poses the following problems due to its high strength and mechanism of strengthening. Specifically, sensitivity to nonmetallic inclusions in the material increases as the strength rises. The concentration of stress by these inclusions lowers the fatigue strength and tends to create inferior durability.
Therefore, improvement of the fatigue characteristics has been attempted to resolve such problems by melting by vacuum induction melting (VIM), then remelting by vacuum arc remelting (VAR) to raise the degree of cleanness of nonmetallic inclusions by controlled reduction of N and O and thereby to reduce the number of nonmetallic inclusions that serve as the origin of fatigue rupture.
The above technology improved the durability to a certain extent. However, the conditions of use of machinery and constructs have become more rigorous in recent years and demands on the strength characteristics of materials have become increasingly severe.
Further improvement of the durability is also demanded to assure the long-term stability of machinery and constructs. This has led to a demand for the development of maraging steel with superior fatigue characteristics for the construction of machinery. Another problem with the conventional production process was the low productivity and the need for expensive, special vacuum arc remelting equipment since vacuum arc remelting was conducted after vacuum induction melting.
The present invention takes note of these problems and has as its object to propose maraging steel with excellent fatigue characteristics and a production process that makes it possible to manufacture the aforementioned maraging steel easily without vacuum arc remelting. This goal is attained by the present invention described below.